1. Field of the Invention
The invention relates to a device for applying solder to a workpiece and more particularly to an apparatus for wave solder treatment of printed circuit boards.
2. Description of the Related Art
Most electronic devices use a circuit board as the fundamental building block for the electronic circuitry which forms the device. With the advent of the printed circuit board (PCB), the board itself actually became part of the circuit, forming conductors for the electrical current between the various electronic components. The circuit board both supports the electronic components and links the components together. Generally, the printed circuit board consists of a non-conductive base, such as fiberglass, coated with a thin layer of a conductor, such as copper, which is etched to form a pattern of electrical conductors. Further advances in PCB technology have resulted in dual sided PCBs, configured so that components are attached on both sides of the boards, and in multilayered PCBs having layers of conductors buried in the non-conductive substrate, which allow intricate and complex circuits to be implemented in a smaller area.
One technique used for soldering the various electronic components to a circuit board substrate is wave soldering. Wave soldering is a continuous process in which circuit boards are positioned in a solder pallet having a solder pattern. The PCB boards positioned in the pallet are then passed over a standing wave of molten solder and the pattern is configured so as to allow molten solder to flow over the regions of the PCB board that are to be soldered. Specifically, the pallet includes apertures which correspond to the areas of the circuit board on which solder is to be deposited and recesses on top of the pallet to house and protect components which would otherwise be exposed to the solder.
In general, solder pallets have a flat bottom surface with the apertures formed in the inner regions of the pallet. During the wave soldering process, the solder wave, which has a constant pressure, is directed towards the flat bottom surface of the solder pallet and the pallet and PCB board assembly is moved relative the wave solder source so that the solder wave travels across the bottom surface of the pallet and wicks up through the apertures containing the components leads to solder the component leads to the PCB board.
When using a solder pallet, the solder wave needs to be tall enough (i.e., pressured enough) so as to wick up through the apertures to make contact with the component leads and the board to result in acceptable soldering of the components leads. However, the solder wave may be pressurized at a level such that when it contacts the flat bottom of the pallet, a large amount of solder can overflow or splash outside of the perimeter of the solder pallet and can even splash over the solder machine or sides of the pallet. This splashed solder can solidify on the solder machine or the pallet and cause machine down times for cleaning, which adds to manufacturing cost.
One prior art solution to this problem is to cut continuous channels in the bottom of the solder pallet to reduce the pressure of the solder wave impacting the bottom surface of the pallet. Specifically, in the prior art, uniform channels are cut from the edge of the apertures to the outside edge of the pallet to reduce the pressure of the solder wave by releasing the solder gases and draining the excess solder. However, this method is not efficient. Since the apertures are connected to the continuous channels, the regions to be soldered on the PCB also experience a reduction in solder flow pressure. The channel effectively reduces the pressure all the way along the path of the solder wave, thereby reducing overflow or splashing adjacent the perimeter of the solder pallet but at a cost of also reducing the pressure at the location of the apertures. In many applications, this results in poor solder connections at the location of the apertures since the reduction in the pressure of the wave at the aperture causes less solder to reach the component leads.
Hence, there is a need for a wave solder pallet which allows for adequate soldering of the component leads but otherwise reduces the quantity of solder that overflows or splashes about the periphery of the solder pallet during the wave soldering process.